Digital pixel sensor (DPS) is one kind of CMOS (Complementary Metal Oxide Semiconductor) image sensors. For this kind of sensor, conversion from analog to digital signal is performed in pixels, and the subsequent data readout and processing are both handled in digital domain. PWM (Pulse Width Modulation) is one kind of DPS. With reference to FIG. 1, pixel structure and operation based on PWM are described below. A typical PWM pixel is composed of a photodiode PD, a resetting transistor MRST, a pixel or column level comparator, and a pixel or column or array level memory (FIG. 1 shows a comparator and a memory both of which are of pixel level). The two input ends of the pixel level comparator are connected with PD node voltage and a predefined reference voltage Vref. Input data of the pixel level memory is provided by a global counter arranged outside of a pixel array. The photodiode PD is reset to the reset voltage Vrst in advance. During pixel integration, the photodiode capacitor is discharged by photocurrent generated by light, thus resulting in drop of the node voltage. The pixel level comparator compares the PD node voltage with the Vref, and if it is decreased to Vref, the output of the comparator Vout transits from high level to low level. This transition signal controls the memory of pixel level to stop “writing” operation and the value of the global counter is stored in the memory. At this time, data stored in the memory is just quantitative value of integration time of the pixel tint, and this value is equivalent to the pulse width of the pixel generated in a time gap from beginning of integration to output toggle of the comparator. This value may be expressed as:
                              t          int                =                                            (                                                V                  rst                                -                                  V                  ref                                            )                        ·                          C              PD                                            I                          p              ⁢                                                          ⁢              h                                                          (        1        )            
where, Iph is photocurrent, and CPD is the capacitance of the photodiode PD. Refer to FIG. 2, value of tint may represent photocurrent value of the pixel and tint is inversely proportional to Iph. As the example in FIG. 2 shows, photocurrent of the photodiode PD under two kinds of light intensity is Iph1 and Iph2 respectively. According to formula (1), corresponding pulse widths are t1 and t2, and then:
                                          t            1                                t            2                          =                              I                          p              ⁢                                                          ⁢              h              ⁢                                                          ⁢              2                                            I                                          p                ⁢                                                                  ⁢                h                ⁢                                                                  ⁢                1                            ⁢                                                                                                      (        2        )            
Assume the maximum and minimum signals which can be detected by the PWM pixel are Iph_max and Iph_min respectively. Then, the DR (Dynamic Range) may be expressed as:
                    DR        =                              20            ×                          log              ⁡                              (                                                      I                                          p                      ⁢                                                                                          ⁢                      h                      ⁢                                                                                          ⁢                      _                      ⁢                                                                                          ⁢                      ma                      ⁢                                                                                          ⁢                      x                                                                            I                                          p                      ⁢                                                                                          ⁢                      h                      ⁢                                                                                          ⁢                      _                      ⁢                                                                                          ⁢                      m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                      )                                              =                      20            ×                          log              ⁡                              (                                                      t                                          ma                      ⁢                                                                                          ⁢                      x                                                                            t                                          m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                      )                                                                        (        3        )            
From the above operation of PWM pixel, it can be understood that: under weak light condition, when the light intensity is below a constant threshold value, photocurrent of the photodiode is so weak (lower than Iph_min), that the node capacitor discharges slowly. Within predefined integration time, the node voltage will not be reduced to Vref, and therefore, no transition signal is generated to perform “write” operation. In other words, detection ability of this structure is restricted by Vref. Under high light intensity, the photodiode has large photocurrent (larger than Iph_max) and the PD voltage rapidly drops to Vref. The counter has no time to generate valid value to be stored into the memory and therefore, information regarding high light intensity is lost. In a summary, under low and high light intensity environment, a typical PWM pixel structure requires higher and lower Vref respectively for shortening or extending tsig for the subsequent processing. Therefore, in this kind of PWM pixel, constant Vref is often not suitable for natural light environment, due to the limited DR. As a result, to obtain a larger dynamic range, some prior art adopts reference voltage which changes with time, and eliminates reset noise and offsets among pixels by multiple sampling. The principle is as follows:
During the exposure, the comparison of reference voltage includes two major periods. The first period is reset sampling period and the second period is integration sampling period. As shown in FIG. 3, exposure time includes a reset sampling period Trs and an integration sampling period Tis. In reset sampling period, Vref linearly rises from Vref_rsl to Vref_rsh, while in integration sampling period, Vref rises from Vref_isl to Vref_ish. In reset sampling and integration sampling periods, the PD voltage match Vref two times at tPD_rs and tPD_is respectively. Therefore, the difference between two time values tPD represents the light intensity, and the differencing operation eliminates reset noise and offset among pixels. This must be done under condition that the ramp changing rate of the reference voltage during reset sampling and integration sampling periods must be exact the same. Otherwise, tPD_rs and tPD_is will not completely eliminate reset noise and offsets, and extra offsets may be introduced. However, the integration sampling period time Tis is generally much longer than reset sampling period time Trs, and Tis is normally 100-500 times as large as Trs. Consequently, it is hard to keep consistent between reference voltage ramp changing rates of the two periods.